Method and apparatus for purging molten metal of gaseous impurities



Dec. 13, 196.6 \/ERGE ET AL 3,291,596

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3,291,596 METHOD AND AirPARATUS FOR PURGING MOLTEN METAL 9F GASEQUS HMPURHTIE Jacques Verge, Metz, France, and Etienne Spire, Westmount, Montreal, Canada, assignors to Institut de Recherches de la Siderurgie Francaise, Saint-Germainen-Laye, Seine-et-Gise, France, a professional institution, and lAir Liquide, Societe Anonyrne pour EEtude et lExpioitarion rles Procedes Georges Claude, Paris, France, a French society Filed Mar. 13, 1964, Ser. No. 351,707 Claims priority, application France, Mar. 14, 1963, 927,979, Patent 1,359,312 8 Claims. (Cl. 7549) This invention relates to the purging of molten metal of gaseous impurities, and is more particularly concerned with an improvement of the process and apparatus disclosed in the United States Patent No. 2,993,780.

The apparatus disclosed in the patent includes a ladle in which a porous plug constitutes a portion of the bottom. The plug is at the bottom of the fused metal within the ladle during the purging operation. An inert purging gas is blown under pressure through the plug and through the metal while a vacuum is maintained in the top portion of the ladle above the metal surface.

The presence of the purging gas in the top portion at a certain overall pressure reduces the partial pressure of the gas which it is desired to remove from the metal, and removal of the undesired gas from the metal is thereby enhanced. The undesired gas also tends to diffuse into ited States Patent the bubbles of purging gas as the latter move through the body of molten metal. The purging gas agitates the molten metal. Portions of the metal are brought from the ladle bottom to the surface. The resulting decrease in static pressure on these metal portions favors release of the undesired gas. Agitation also breaks the continuity of a layer of slag which otherwise may prevent contact between the metal surface and the evacuated space above.

For a fixed flow rate of the purging gas, as determined under standard conditions of temperature and pressure, the agitation effect of the purging gas increases as the gas pressure in the top portion of the ladle is reduced. Spattering or overflowing of the metal may be caused by a constant stream of purging gas as pressure within the ladle decreases during evacuation.

The primary object of the invention is the provision of a hi h rate of removal of the undesired gas during the initial stages of purging without causing spattering, foaming, or overflowing of the metal when a high vacuum is reached later.

Another object is the shortening of the purging cycle.

A further object is a reduction in the necessary capacity of the vacuum equipment connected to the ladle.

It has been found that there exists a critical relationship between the gas pressure above the surface of a body of molten metal, and more particularly ferrous metal, and the maximum rate at which a purging gas may be passed upward through a unit area of the metal surface. When the rate of purging gas flow (measured under standard conditions of temperature and pressure) is only slightly below the critical value at the prevailing gas pressure in the ladle, the purging gas is released from the metal surface smoothly and without a significant amount If the critical flow rate is exceeded, the metal spatters and foams.

The permissible maximum flow rate varies with the composition of the melt and of the purging gas, with the dimensions of the container, with the degree of dispersion of the purging gas in the molten metal, and with similar factors. It thus may vary from one installation to an- Bihlfthh Patented Dec. 13, E966 ice other or from, one type of operation to another in the same equipment, but it is remarkably constant when the same type of melt is purged by any purging gas in the same ladle or other container. It is readily possible to plot a graph correlating the maximum permissible flow rate of inert purging gas to the internal gas pressure in the container from a few points empirically determined under normal operating conditions.

Based on this finding of a predetermined relationship between the gas pressure in the container and the flow rate of the purging gas, the invention in one of its aspects mainly consists in generating a signal representative of the gas pressure in the top portion of the container, and in controlling the flow rate of the purging gas in response to the signal generated. In another aspect, the invention resides in an apparatus for generating the signal, and in flow control means for controlling the flow of purging gas accordingly.

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawings in which:

FIG. 1 shows an apparatus of the invention in elevational, partly sectional view:

FIG. 2 is a graph correlating the gas pressure in the apparatus of FIG. 1 with the flow rate of a purging gas;

FIG. 3 graphically illustrates the change of gas pressure in the apparatus of FIG. 1 when the apparatus is being evacuated;

FIG. 4 is a graph illustrating the operation of a modified apparatus of the invention in the manner of FIG. 2; and

FIG. 5 shows another apparatus of the invention in elevational, partly sectional view, and includes a control circuit represented in a conventional manner.

Referring now to the drawing in detail, and initially to FIG. 1, there is shown a vacuum chamber 1 consisting of three parts which are releasably sealed to each other. The top of the chamber is formed by a dished cover in and, the bottom by a pan 1c. The cover and the pan are connected by an upright wall 1b. A sight glass 2 in the cover In permits inspection of the chamber contents.

A suction pipe 3 connects the pan 1c with a vacuum pump, not shown. A cooling jacket 4 sourrounds a portion of the pipe 3. A jacket 4 receives cooling water through a supply pipe 5, and the spent cooling water is discharged through an outlet pipe 5.

A ladle 6 of conventional frustoconical shape is supported in the chamber 1 on heavy I-beams 7. The bottom wall of the ladle 6 is partly constituted by a porous plug 8 permeable to gas, but not permitting passage to the molten metal 6' in the ladle. A feed pipe 9 is connected to the plug 8 in the illustrated operative condition of the apparatus. The pipe 9 passes outward through the pan 10. It receives nitrogen or argon under uniform pressure through a supply pipe 10 provided with a main shut-off valve 11. The pipes 9 and 10 are connected by five conduits 12, 13, 14, 15, 16 which are arranged in parallel. The conduits 12-16 are equipped with respective manually adjustable control valves 1211-1612. The conduits 12, 13, 14, 15, moreover have solenoid valves 12a15a arranged in series with the corresponding control valves. The solenoid valves 12a to 15a are actuated by respective relays to and a source of electrical current, not shown in the drawing. The relays are triggered by corresponding aneroid adjustable pressure switches 12d to 15d.

The electrical control circuit will be understood further to include such conventional elements as main switches, fuses, transformers to reduce line voltage to a desired lower operating voltage for the relays, and the like, and such conventional elements, while not explicitly illustrated, will be understood to be present in the disclosed apparatus.

The manner in which the apparatus of FIG. 1 is op erated will be described with reference to FIG. 2 which is a graph of the flow rate Q of purging nitrogen or argon in arbitrary units of mass per unit time independent of instaneous temperature and pressure versus gas pressure P within the chamber 1 in millimeters mercury.

Critical conditions of maximum permissible flow rate Q at the prevailing pressure P are represented by the obliquely sloping characteristic line B, F, H, J, C. The lowest pressure p obtainable with the vacuum pump on hand corresponds to point C on the characteristic line, and the extrapolated portion of the characteristic toward pressure is represented by a broken line.

The four pressure switches 12d-15d are set to trigger the associated relays at pressures within the suction pipe 3 near the chamber 1 corresponding to the points B, F, H, J, and to deenergize the relays at lower pressures. The manually adjusted control valves 12b to 1612 are set for approximately equal open flow sections. The solenoid valves 12a-15a are normally closed by their return springs.

A ladle 6 charged with molten ferrous metal to be purged is placed on the supporting beams 7, and the chamber is sealed about the ladle. The vacuum pump is started, and the gas withdrawn from the chamber 1 is cooled by the water in the jacket 4 for better efiiciency of the pump. The switches 12d to 15d are set for pressures lower than the normal ambient pressure of 760 mm. Hg. The relays 12c to 15c therefore initially energize the corresponding solenoid valves 12a to 15a, and nitrogen or argon is admitted from the supply pipe 10 to the feed pipe 9 as soon as the main shut-off valve 11 is opened. Opening of the main valve is preferably delayed until the pressure in the chamber 1 is at least somewhat lowered.

The rate of gas flow with all solenoid valves open is indicated in FIG. 2 by the line AB. It is constant if the purging gas is supplied from the valve 11 at a constant pressure. A supply of purging gas at practically constant pressure may be provided by using a large gas storage tank as a source of gas, by replenishing the gas in the storage tank in a known manner to maintain its pressure, or simply by incorporating a pressure reducing section in the valve 11, and by making the initial gas pressure in the storage tank much higher than that for which the reducing section is set. The how of purging gas into the chamber 1 is lower at all times than the pumping capacity of the vacuum equipment.

The first pressure switch 120? opens its contact when the pressure in the chamber 1 drops to a value of B. The resulting sudden closing of the solenoid valve 12a reduces the flow rate Q to E. The supply of purging gas to the porous plug 8 then continues at a constant rate until the chamber pressure drop to F, whereupon the switch 13d is opened. This cycle of events is repeated twice more with switches 14a and 15d and the associated relays and valves until the flow rate Q is reduced step by step to the level K-C which is maintained by the permanently open manual control valve 16b until the main valve 11 is shut.

When the degassing of the metal charge 6' is completed, the non-illustrated vacuum pump is stopped, and the chamber 1 is vented to the atmosphere in a conventional manner through a non-illustrated valve associated with the pump. The chamber 1 is dismantled. The feed pipe 9 is disconnected from the plug 8, and the ladle 6 is transferred to another operation.

As long as the vacuum pump and the associated apparatus shown in FIG. 1 operate normally, the pressure drops in the chamber 1 in a manner which is precisely predictable after the first run. FIG. 3 is a plot of internal chamber pressure P in millimeters mercury as a function of time t in arbitrary units. The fully drawn curve is representative of every run of the apparatus of FIG. 1 under normal conditions. The pressure drops from an initial value of 760 mm. Hg at a gradually decreasing rate toward a value C characteristic of the pump employed, and asymptotically approaches this value which is somewhat higher than zero. The pressure P in the chamber 1 is uniquely related to the time elapsed after starting of the vacuum pump.

This fact permits the apparatus illustrated in FIG. 1 to be modified and simplified without loss of function. The pressure switches 12d15d may be omitted and the relays 12c15c may be replaced by the several decks of a rotary timing switch driven by a synchronous motor coupled to the vacuum pump motor or a similar timing device synchronized with the operation of the pump.

The operation of the modified apparatus is illustrated in FIG. 4 which is a plot of the flow rate Q of purging gas versus time 1, both variables being represented in arbitrary units. The contacts of the timing switch are set for the times t 't t and 1 after the time t at which operation of the switch motor and of the pump motor is started. The curve B'FH'JC' is the characteristic of the apparatus of FIG. 1 in terms of time and maximum permissible flow rate of inert purging gas. The several levels to which the rate of gas flow is reduced step-by-step are represented by the straight lines AB', E'F', H'G, IJ',

and K'C' respectively as will be evident from the preceding discussion of FIG. 2.

The apparatus illustrated in FIG. 5 is identical with that shown in FIG. 1 with respect to the chamber 1, the suction pipe 3 and its cooling jacket 4, the non-illustrated vacuum pump, the ladle 6 and the feed pipe 9. The admission of purging gas to the latter pipe is controlled by a valve 18 whose threadedly movable valve member, as in a needle valve, is actuated :by an electric motor.

The pressure signal according to which the valve 18 is operated is generated by a low pressure gauge 19 of the diaphragm type provided with an electrical transmitting device such as a resistor with a sliding contact actuated by the motions of the diaphrgam. An aneroid switch 20 and the gauge 19 communicate with the suction pipe 3. At pressures above a fixed value, the movable contact S of the aneroid switch is connected to a fixed contact M. A source 21 of constant voltage is connected to the contact M by a rheostat 22 having a sliding contact. At low pressures within the pipe 3, the movable contact S of the aneroid switch 20 is connected by a fixed contact N of the switch to one terminal of the gauge 19. The other gauge terminal and one pole of the voltage source 21 are grounded.

The movable switch contact S is conductively connected to the sliding contact U of a voltage dividing resistor 23 one tenrninal of which is grounded. A tap L of the resistor 23 is connected with one input terminal of an amplifier 25. The other input terminal of the amplifier is connected to the sliding contact I of a potentiometer 24. One terminal r of the potentiometer is grounded, and the other terminal u is connected to one pole of a source of constant voltage 27 through a rheostat 28 having a sliding contact y. The other pole of the voltage source 27 is grounded.

The output of the amplifier 25 is fed to a motor 26, and rotates the motor in one direction or the other depending on the direction of current flow between the input terminals. The motor 26 actuates threaded movement of the valve needle in the valve '18 and pivoting movement of the sliding contact I in the potentiometer 24.

The characteristic of the apparatus shown in FIG. 5 is represented by the line ABFHJC in FIG. 2 whereby B corresponds to a pressure of 500 millimeters mercury pressure, and C to a pressure of 20 millimeters. The voltage source 21 together with the rheostat 22 and the voltage dividing resistor 23 in elfect constitute an adjustable reference level to which the potentiometer 24- is set by the motor 26 to minimize an error signal furnished by the amplifier 25 in a feedback control loop operating in a conventional manner at the relatively high initial pressure in the chamber 1.

As long as the pressure in the suction pipe 3 is above the value B in FIG. 2, the aneroid switch 20 connects the contacts M and S. g The flow of purging gas through the valve 18 is therefore set to a constant value which is determined by the positions of the sliding contacts X and U, and by the resulting potential between the tap L and the terminal R of the resistor 23.

When the pressure in the pipe 3 drops below the value B, the potential between the tap L and the terminal R of the resistor 23 varies with the output of the Pirani gauge 19, and is a direct measure of the maximum amount of purging gas that may be passed through the valve 18 into the feel pipe 9. When the sliding contact y of the rheostat 28 is properly adjusted, the potential between the terminals r and l of the potentiometer 24- is a unique function of the position of the movable contact I which is mechanically coupled to the motor 26 and to the valve 18. The voltage between the terminals l and r is a direct function of the desired flow rate of purging gas.

The input voltage of the amplifier between the tap L and the movable contact I thus represents the difference between the'maximum permissible flow rate corresponding to the pressure prevailing in the conduit 3, and the actual flow rate determined by the common position of the valve 18 and of the movable contact I. When the input voltage is different from zero, the motor 26 displaces the valve 18 and the moving contact Z until a condition of zero input voltage is established. When proper proportionality between the chamber pressure and the flow rate of the purging gas has once been set on the sliding contacts U and y, the apparatus automatically maintains the maximum permissible rate of purging gas flow after an initial delay determined by the switch-over pressure of the aneroid switch 20.

It will be appreciated that the switch 20- with its movable membrane which carries the contact S may be replaced by a timing switch synchronized with the pump motor in the manner discussed hereinabove with reference to FIG. 4. Except for the actuation of movement of the contact S, the mode of operation of the apparatus is not altered, and the result obtained is the same. The period prior to switching over is normally of the order of a few minutes in good industrial practice, whether the switchover be controlled by time or according to the prevailing pressure in the pipe 3.

A melt of metal is purged of undesired gaseous constituents by a stream of purging gases at a rate which is directly related to the flow rate of the purging gas. The apparatus of the invention illustrated in FIG. 5 permits the maximum flow rate to be maintained which is consistent with the prevention of foaming, spattering, or overflowing of the melt. The removal of the undesired gas is therefore achieved as rapidly as possible.

The above described method and apparatus have been used for purging a SO-ton ladle of molten steel. The vacuum pumping equipment comprised ejectors having a pumping capacity of 100 kilograms per hour at a pressure of 1 mm. Hg.

At the start, that is at the time t=0, the pressure was p=760 mm. Hg. The pressure was lowered and nitrogen introduced at a constant rate of 500 litres per minute.

At the time t=50 seconds, the pressure was p=500 mm. Hg, the rate of nitrogen flow being still of 500 litres per minute. From that time forth the rate of flow of purging gas was under control of the pressure.

At the time t=4 minutes, the pressure was 015 mm. Hg, the rate of nitrogen flow being then 100 litres per minute.

Between the pressures of 500 and 0.5 mm. Hg the rate of flow decreased from 500 to 100 litres per minute according to a linear function. The operation was then carried out but the pressure did not decrease any more 5 because ejectors were working at their maximum capacity. Consequently the rate of flow was maintained at litres per minute until the end of operation.

The stepwise reduction in the flow rate of the inert purging gas illustrated in FIG. 2 with reference to the apparatus of FIG. 1 is preferred in installations in which the greater simplicity of the apparatus is of paramount importance.

It should be understood, of course, that the foregoing disclosure relates only to preferred embodiments of the invention, and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purpose of the disclosure which do not constitute departures from the spirit and scope of the invention set forth in the appended claims.

What is claimed is:

1. A method of purging a body of molten metal of impurities which comprises:

(a) enclosing said body in a container in such a man ner that said body has a free surface in the closed container;

(b) feeding a stream of a purging gas to a portion of said body downwardly spaced from said free surface while simultaneously withdrawing gas from a portion of said container above said surface;

(c) generating a signal representative of the gas pressure in said container portion; and

(d) controlling the flow rate of said stream in response to the signal generated and in direct relation to said gas pressure.

2. A method as set forth in claim 1, wherein gas is withdrawn from said container portion at a rate substantially greater than the rate of feeding of said stream, whereby the pressure in said container portion decreases, and said stream is controlled 'to produce a flow rate decreasing with decrease of said pressure.

3. A method as set forth in claim 2, wherein said signal is generated after said gas pressure has been decreased to a predetermined value, and said stream of purging gas is fed to said portion of said body at a substantially constant rate prior to said signal.

4. A method as set forth in claim 2, wherein the flow rate of said stream is decreased stepwise responsive to said signal while said gas pressure decreases substantially continuously.

5. A method as set forth in claim 2, wherein gas is withdrawn from said container portion at a predetermined rate, and said signal is a time signal.

6. A method as set forth in claim 1, wherein the purging gas stream enters the molten metal body, and passes therethroughto said free surface, in the form of bubbles.

7. An apparatus for purging a body of molten metal of impurities comprising, in combination:

(a) a container enclosing the body of metal to be purged and a free surface of said metal body dividing a top portion of said container from a bottom portion;

(b) a conduit for feeding a stream of a purging gas to said bottom portion;

(c) suction means for withdrawing gas from said top portion;

(d) signal generating means for generating a signal representative of the gas pressure in said top portion; and

(e) fiow control means in said conduit for controlling the flow rate of said stream in response to the signal generated.

8. An apparatus for purging a body of molten metal of impurities comprising, in combination:

(a) a container enclosing the body of metal to be purged, and a free surface of said metal body dividing a top portion of said container from a bottom portion,

7 '8 (l) porous plug means constituting a part of the the flow rate of said stream in response to thesigbottom portion of said container; nal generated. (b) a source of purging gas under pressure; (0) a conduit for feeding a stream of the purging gas References Cited y the Examine! from the purging gas source to the porous plug 5 UNITED STATES PATENTS mans? 3,071,458 1/1963 Finkl 75 49 g t ggo means for Wlthdrawmg g m s p 3,188,198 6/1965 Moore 75 49 3,236,635 2/1966 Finkl 7549 (e) signal generating means for generating a signal representative of the gas pressure in said top por- 10 DAVID L RECK Primary Examiner tion', and

(f) flow control means in said conduit for controlling H. W. TARRING, Assistant Examiner. 

1. A METHOD OF PURGING A BODY OF MOLTEN METAL OF IMPURITIES WHICH COMPRISES: (A) ENCLOSING SAID BODY IN A CONTAINER IN SUCH A MANNER THAT SAID BODY HAS A FREE SURFACE IN THE CLOSED CONTAINER; (B) FEEDING A STREAM OF A PURGING GAS TO A PORTION OF SAID BODY DOWNWARDLY SPACED FROM SAID FREE SURFACE WHILE SIMULTANEOUSLY WITHDRAWING GAS FROO A PORTION OF SAID CONTAINER ABOVE SAID SURFACE; (C) GENERATING A SINGLE REPRESENTATIVE OF THE GAS PRESSURE IN SAID CONTAINER PORTION; AND (D) CONTROLLING THE FLOW RATE OF SAID STREAM IN RESPONSE TO THE SIGNAL GENERATED AND IN DIRECT RELATION TO SAID GAS PRESSURE.
 7. AN APPARATUS FOR PURGING A BODY OF MOLTEN METAL OF IMPURITIES COMPRISING, IN COMBINATION: (A) A CONTAINER ENCLOSING THE BODY OF METAL TO BE PURGED AND A FREE SURFACE OF SAID METAL BODY DIVIDING A TOP PORTION OF SAID CONTAINER FROM A BOTTOM PORTION; (B) A CONDUIT FOR FEEDING A STREAM OF A PURGING GAS TO SAID BOTTOM PORTION; (C) SUCTION MEANS FOR WITHDRAWING GAS FROM SAID TOP PORTION; (D) SIGNAL GENERATING MEANS FOR GENERATING A SIGNAL REPRESENTATIVE OF THE GAS PRESSURE IN SAID TOP PORTION; AND (E) FLOW CONTROL MEANS IN SAID CONDUIT FOR CONTROLLING THE FLOW RATE OF SAID STREAM IN RESPONSE TO THE SIGNAL GENERATED. 